1. Field of the Invention
The present invention relates to a polymerization inhibitor composition and a method of inhibiting polymerization of distillable monomers in liquid and evaporated/condensed phases with the polymerization inhibitor composition. The polymerization inhibitor composition is useful for inhibiting polymerization of the distillable monomers during manufacture, purification (e.g., distillation), handling, and storage thereof.
2. Description of the Related Art
Acrylic acid-type monomers include monomers having a structural fragment,
wherein
represent portions of covalent bonds. The acrylic acid-type monomers are widely used in chemical industry to prepare polyacrylate-type polymers (e.g., poly(acrylic acid)s and poly(methacrylic acids)), which are useful as, among other things, additives (e.g., for use in rubbers, coatings and adhesives) and ingredients in fibers and other articles. A major problem with the acrylic acid-type monomers is premature polymerization. In the absence of sufficient amounts of polymerization inhibitors, acrylic acid-type monomers undesirably polymerize during manufacture, purification, handling, and storage operations.
An especially difficult problem has been premature polymerization of evaporated and condensed acrylic acid-type monomers in headspaces of vessels and piping having headspaces. For example, purification by reduced pressure distillation comprises distilling an acrylic acid-type monomer from a distillation pot having a volume of monomer and headspace and through a distillation column (headspace). During distillation a portion of the acrylic acid-type monomer evaporates and eventually condenses on certain structures (i.e., “cold” spots) within the column. If the condensed monomer lacks a polymerization inhibitor, it will polymerize and thereby foul structures in the column, especially structures that are more not in direct liquid communication with the distillation pot and from column zone receiving splashes therefrom. Eventually the distillation operation must be shut down and the fouled column cleaned, an expensive and time-consuming maintenance operation. To inhibit polymerization, the chemical industry has added polymerization inhibitors to batches of the monomers before subjecting the batches to these operations or, less desirably, physically added such polymerization inhibitors during the operations, but with less than desirable results.
For example, some such prior art polymerization inhibitors are known from U.S. Pat. No. 4,210,493, which mentions nitrosobenzene and 4-nitrosophenol, among others and optionally physically adding the same to plates in distillation columns during distillations; U.S. Pat. No. 5,272,231, which mentions nitric oxide, among others; U.S. Pat. No. 5,504,243, which mentions certain N-oxyl compounds and certain nitroso compounds, among others; U.S. Pat. No. 5,888,356, which mentions certain N-oxyl compounds and certain nitroso compounds; U.S. Pat. No. 7,241,915 B2; U.S. Pat. No. 7,319,167 B2, which mentions a method of physically adding certain such polymerization inhibitors to a condensate or a reflux liquid during distillation; U.S. Pat. No. 7,319,168 B2; and U.S. Pat. No. 7,388,109 B2. Also see WO 2009/032427. But such prior art polymerization inhibitors are unsatisfactory for evaporating and condensing with, and thereby inhibiting polymerization of condensates of, acrylic acid-type monomers on the structures in distillation columns. Either they are insufficiently volatile to evaporate to the structures or they are overly volatile and do not condense on the structures with condensed acrylic acid-type monomers.
The overly-volatile prior art polymerization inhibitors comprise compounds that readily evaporate at vapor pressures and temperatures suitable for distilling acrylic acid-type monomers, but either do not condense with condensed acrylic acid-type monomers on the structures in the columns or condense in amounts below those required to effectively inhibit polymerization of the acrylic acid-type monomer condensates. Examples of the overly-volatile polymerization inhibitors are gases such as oxygen gas and nitric oxide gas, which have been used to inhibit vapor-phase polymerization but do not condense in such reduced pressure distillation columns.
The insufficiently-volatile prior art polymerization inhibitors comprise two types of inhibitors. One such type of insufficiently-volatile polymerization inhibitor comprises compounds that, at vapor pressures and temperatures suitable for distilling acrylic acid-type monomers, are substantially captive to the distillation pot batch of monomer to which they are added. The other type of insufficiently-volatile polymerization inhibitor comprises compounds that, before evaporating, react in situ to form captive derivative polymerization inhibitor compounds. Examples of the former type of insufficiently-volatile polymerization inhibitor are 4-nitrosophenol, 2-methyl-4-nitrosophenol, N-nitrosodiphenylamine and salts thereof, N-nitrosophenylhydroxyamine and salts thereof, and phenylnitroxide. An example of the latter type of insufficiently-volatile polymerization inhibitor is a lone inhibitor nitrosobenzene, which, before evaporating, reacts in situ during such distillations to form phenylnitroxide. Substantial amounts of the insufficiently-volatile polymerization inhibitors, or insufficiently-volatile derivatives of polymerization inhibitors such as phenylnitroxide, remain in distillation pots during such reduced pressure distillation operations and essentially do not inhibit polymerization of the evaporated and condensed acrylic acid-type monomers on the structures in the columns.
The chemical industry desires superior polymerization inhibitor compositions comprising a captive polymerization inhibitor and a fugitive polymerization inhibitor each independently capable of inhibiting polymerization of distillable monomers (e.g., acrylic acid-type monomers and other olefinic monomers) in vessels (e.g., distillation pots) and piping and the fugitive polymerization inhibitor preferably further capable of evaporating into headspaces in the vessels and piping and into distillation columns, condensing in a polymerization inhibiting amount with the distillable monomers, and inhibiting polymerization of the evaporated and condensed distillable monomers on structures in the headspaces (e.g., distillation columns), including structures distal from unevaporated acrylic acid-type monomers remaining in the vessels or pipes.